Power semiconductor devices are desired to be of low power consumption in order to reduce power loss. For instance, the power consumption of a vertical power MOSFET, which is one of the power semiconductor devices, greatly depends on the electric resistance of its conduction layer (drift layer) determining the on-resistance. The amount of impurity doping determining the electric resistance of this drift layer cannot exceed a maximum limit depending on the breakdown voltage of the pn junction between the base layer and the drift layer. Thus, there is a tradeoff between the device breakdown voltage (avalanche withstand capability) and the on-resistance, and devices have been optimally designed under this tradeoff. On the other hand, this tradeoff has an intrinsic limit dependent on the device material and structure. Technology development overcoming this limit is the way to realizing low power consumption devices beyond existing power semiconductor devices.
For instance, as a power semiconductor device with the aforementioned tradeoff improved, a MOSFET having a super junction structure (SJ structure) is known in which p-type pillars and n-type pillars are periodically buried in the drift layer. In the SJ structure, by equalizing the amount of charge (amount of impurity) contained in the p-type pillar with that contained in the n-type pillar, a non-doped layer is artificially produced to hold a high breakdown voltage. Furthermore, a current is passed through the highly doped n-type pillar to realize a low on-resistance close to the material limit.
Known methods for forming a SJ structure in the drift layer include the method of forming p-type pillars and n-type pillars in a semiconductor layer by stacking epitaxial growth layers which are selectively doped with impurity by ion implantation, and the method of forming trench grooves in an n-type semiconductor layer and filling the trench grooves with p-type semiconductor to form p-type pillars. In the former, although technically less difficult, it is necessary to repeat ion implantation and epitaxial growth a plurality of times, which causes the problem of high process cost. On the other hand, in the latter, it is necessary to grow a high-quality semiconductor crystal in the trench groove with high aspect ratio, which involves a high level of technical difficulty. However, the latter method is promising for significantly reducing the process cost.
On the other hand, a high breakdown voltage is desirable for stable operation of a power semiconductor device. In particular, it is important to ensure a high breakdown voltage in the terminal portion around the device where the symmetry of the structure is broken. For instance, JP-A-2008-078282 discloses a semiconductor device in which the device breakdown voltage is increased by elongating the p-type pillar in the terminal portion of the MOSFET. However, the problem in using the method for forming a SJ structure by filling trench grooves is that it is difficult to increase the breakdown voltage by varying the length of the p-type pillars exclusively in the terminal portion.